The present invention relates to a technique for producing holographic patterns and more particularly to the apparatus and method for practicing this technique and the products which result.
It is known to use interferometry to expose light sensitive material (photoresist) so as to harden that material in specific locations. When the unhardened portions are removed, the remaining material forms patterns of lands and grooves which correspond to interference patterns and which can therefore be used to produce holographic products. To that end, the patterns initially formed in the photoresist are processed so that they can then be embossed in metal or plastic. The resulting embossings are used as shims for transferring these patterns onto the final holographic product, such as sheets of paper, plastic film, or the like.
In performing the initial exposure, the interferometric illumination had to remain stationary at each desired location for a sufficient period to harden the photoresist at that location. The illumination would then be moved to the next location and the exposure repeated there. This movement was accomplished by appropriately displacing the interferometer xe2x80x9cheadxe2x80x9d, or the substrate bearing the photoresist. The required dwell time at any particular exposure location was of the order of magnitude of 1 millisecond.
Unfortunately, a dwell time of that duration was frequently incompatible with unintentional displacements of the interferometer head and/or the photoresist-bearing substrate. Such unintentional displacements can be caused by environmental factors, such as vibrations induced by the nearby passage of vehicles, or by other vibration-producing equipment. They can also be caused by the functioning of the exposure-producing equipment itself. Specifically, since the displacement between consecutive photoresist exposure locations took place intermittently, between exposure at one location and the next, the starting and stopping of this intermittent displacement, in itself, gave rise to vibrations in the equipment. As a result, the stationary dwelling of the illumination at each location was compromised and the resulting, interference pattern was degraded. In turn, this also caused degradation of the holographic effects in the end product.
In practice, even vibrations of small amplitude could lead to serious degradation, because of the high degree of positional precision required to achieve correct interferometric exposures.
Efforts to overcome this problem by using more massive photoresist supports, or more firmly mounted interferometer heads not only led to unwanted complexity, but were sometimes counterproductive. Thus, the more massive the supports, the more difficult it became to displace them without inducing increased start-stop vibrations.
The same problems also tended to limit the size of the surface on which the initial exposures could be performed. In turn, this meant that large holographic surfaces had to be built up from multiple small surfaces placed side-by-side. That caused the appearance, in the final holographic product, of seams which are considered visually objectionable.
Accordingly, it is an object of the present invention to overcome one or more of the problems described above.
It is another object to provide a technique for producing holographic products which is less subject than the prior art to vibration problems.
It is still another object to produce holographic products which are free of seams over substantially larger areas than heretofore.
These and other objects which will appear are achieved as follows.
In accordance with the present invention, a pulsed laser beam is projected interferometrically onto a workpiece, so as to consecutively form interference patterns on selected spots of that workpiece. The beam intensity and the workpiece material are so chosen that this material is ablated in lines which correspond to the illuminated lines of the laser interference pattern. The workpiece and laser beam are displaced relative to each other, so that the consecutive spots are formed at different locations on the workpiece. In this way, there is formed on the workpiece a set of ablation patterns which collectively correspond to a desired overall holographic pattern, or holographic imagery. We have found that the pulsed laser projection on each individual spot can be of extremely short duration, so short that any displacement of that spot on the workpiece due to vibration of either that workpiece or the laser, or both, will be too small to appreciably degrade the interference pattern created at that spot. Indeed, we have discovered that it is even possible to intentionally keep the workpiece and the laser in continuous movement relative to each other, and still create no appreciable degradation of the resulting interference patterns and therefore also no degradation of the ultimate holographic product.
Accordingly, our technique has numerous advantages over the prior art.
In our technique, the laser pulses can have a duration of the order of 6 to 10 nanoseconds, which is some 100,000 times shorter than the 1 milliseconds exposure previously used for photoresists. Obviously, no appreciable displacement of workpiece relative to laser beam can take place during such a period of only a few nanoseconds. Consequently, our technique does not suffer from degradation due to vibration effects and cat be applied to large surfaces. Our technique does not necessarily require intermittent, start-stop movements of the workpiece relative to the laser beam, but can be carried out with continuous relative movement. Our technique operates; more rapidly, since the much longer exposure times required for photoresists are essentially eliminated, and our technique also does not require the chemicals and xe2x80x9cwet chemistryxe2x80x9d involved in using photoresists.